Centrifuges are generally known in the art. They are used, mainly in the chemical, the pharmaceutical and the food industry, for separating, in suspensions, i.e. substances having a liquid and a solid component, the solid phase from the liquid phase and for drying.
Generally, conventional centrifuges comprise a drum with a filter arranged in the drum. The filter can be configured as a rigid metal filter. The gap between the filter and drum wall is also referred to as the annular space. The region inside the filter is referred to as the working space.
In conventional centrifuges, the suspension is first loaded into the working space. This is conventionally done through the drive shaft which is hollow in its configuration, thus allowing it to be used as a loading shaft. The drive shaft is furthermore fixedly connected to the drum base and is used to drive the drum. Conventionally, the drive shaft is mounted horizontally.
The suspension is loaded into the working space as the drum rotates. As a result of the forces acting on the suspension in the radial direction, for example centripetal force, or the forces of inertia resulting therefrom, for example centrifugal force, the suspension is pressed outward against the filter. An appropriately high centrifugal force produces a stable liquid ring. This produces a suspension ring on the filter. The liquid phase then passes through the filter into the annular space and is discharged, whereas the solid phase remains in the working space.
In conventional centrifuges, the solid phase of the product clings tight to the filter after the liquid phase has escaped. The solid phase can in this case have a residual liquid content of up to 30%. The product which clings tight to the filter is in this state also referred to as a cake or ring cake, product cake or filter cake.
The centrifuged product having a high residual liquid content is, in the form present after centrifuging, generally not optimally suitable for onward conveyance for a further “drying” process step. It has been found to be particularly advantageous to dry the product directly in the working space. This eliminates the need to introduce product which is still moist and awkward to convey into a drying space via a transfer unit. In addition, in the case of toxic products, this reduces the risk for the personnel involved. Centrifuges in which a product is centrifuged and dried in the same working space are also referred to as centrifugal dryers.
In conventional centrifugal dryers, the cake must be blasted from the filter prior to drying. Provided for this purpose are swirl nozzles and drum base openings which open out into the annular space. The annular space itself is divided into a plurality of sections by webs, each section having a drum base opening. Furthermore, provision is conventionally made for the swirl nozzles to be able to be brought up to the drum base openings from the outside. A generally gaseous fluid is then injected at high pressure into the annular space through the swirl nozzles. The fluid then moves in the opposite direction through the filter and removes the solid phase of the product, which is pressed into the filter by the centrifugal forces, from the filter. This process is also referred to as blasting of the filter cake. Optionally, a plurality of swirl nozzles can be provided, so the swirl nozzles inject the fluid into the annular space simultaneously, or else there can be provided just one swirl nozzle which successively injects the fluid into the individual sections and thus blasts the filter cake piece by piece.
Blasting of the filter cake is followed by drying of the product. Drying is conventionally carried out by means of either fluidised bed drying or fixed bed drying.
During fluidised bed drying, typically either a stop-and-go process or a continuous process is applied. In the case of the stop-and-go process, a hot drying fluid is injected into the working space through the drum base openings by means of the swirl nozzles. The drum is then rotated further by a specific degree and a further shot of drying fluid is injected into the working space. Thus, the product is dried by the hot gas and mixed up by the successive rotation of the drum in such a manner that the product dries as uniformly as possible.
During continuous drying, the swirl nozzles are not brought quite up to the drum base; instead, a minimal gap is left between the nozzles and the drum base. The drum then rotates continuously at slow speed and an appropriate regulating system of the centrifugal dryer causes the swirl nozzles to inject the drying fluid whenever a drum base opening is situated before the swirl nozzle outlet. To simplify the regulating, the drum base openings are conventionally formed as slots. In this way, even during continuous drying, the product is dried by the drying fluid and repeatedly mixed up by the continuous rotation of the drum, so drying is carried out as uniformly as possible.
During fixed bed drying, the product cake is initially not blasted. Instead, a hot drying gas is introduced into the working space, which flows through the product cake from the inside outward, i.e. from the working space in the direction of the annular space and thus deprives the product cake of moisture. The product cake is thus dried in its ring form and only then is it detached from the filter. This can also be done, for example, by blasting or by inverting the filter in the case of an inverting filter centrifuge.
Following drying, the dried product, which generally assumes the form of a powder, can be removed from the working space and processed further.
However, in the above-described conventional processes, the processing of specific products is problematic. In particular in products having a broad grain size spectrum and a high fine grain content, centrifuging is seriously impeded.
Sedimentation of the relatively large product components takes place as early as during loading. Owing to their differing mass-to-surface area ratios, the relatively large product components move rapidly outward against the filter. However, the fine component initially floats in the liquid and is deposited outward against the filter more slowly. In this case, the fine product components clog the gaps between the relatively large product components, in many cases preventing the liquid phase from flowing away through the capillary between the relatively large product components. During centrifuging the liquid phase then flows only extremely slowly or even not at all. Increasing the rotational speed of the drum does not solve this problem either. The problematic products thus yield after centrifuging a residual liquid content of the product of up to 70%.
In addition, during drying by means of fluidised bed drying of the above-mentioned products having a broad grain spectrum and a high degree of moisture, the product soon forms lumps. During conventional fluidised bed drying, product components, which are moved upward as a result of the rotation of the drum, continually roll down onto the drum base along the remaining product components. The tendency of the product to form lumps is thus greatly promoted, as during the downward rolling relatively small product particles cling to relatively large product particles and increasingly large lumps are thus formed. However, the tendency of the product to form lumps during drying has significant drawbacks. Thus, firstly, the relatively large lumps cannot be dried satisfactorily, as they remain very moist on the inside; secondly, a product which has formed lumps has very poor suitability for further processing.
During conventional fluidised bed drying, what are known as drying cracks frequently form in the product cake during drying. Obviously, escaping of the drying gas through these cracks is promoted owing to the relatively low resistance, so the bulk of the drying gas escapes through the drying cracks without passing through the product per se and having a drying effect. On the one hand the drying gas is not efficiently used in this way, on the other hand the cake cannot be dried uniformly. In addition, high-heat regions, in which the product can become damaged or undesirable chemical reactions occur, are formed in the environment of the drying cracks.
This may create the need for additional and essentially superfluous finishing of the product in order to obtain product consistency which can be processed further.
Furthermore, the filters, in particular metal filters, cannot be produced with as small a mesh size as may be desired. The minimum mesh size is currently about 10 μm. In the case of products having a high fine content, i.e. approximately 20% of the product has a grain size of less than 10 μm, the bulk of the product is lost during the conventional drying process. Specifically during fluidised bed drying, the fine component is constantly atomised and escapes together with the drying gas into the annular space through the filter. Thus, in the case of the conventional processing methods, a significant proportion of the product is frequently lost.
Finally, in the case of specific products, strict conditions are placed on the type and manner of processing. Thus, for example, a maximum temperature of the drying gas may be defined, as a higher temperature would result in damage to the product or undesirable chemical reactions. Also, in many cases a very low maximum residual liquid content of approximately 1% is defined, it is almost impossible to adhere to the conditions using the conventional processes. This is particularly the case with products in the food sector and from the chemical and pharmaceutical sector.
A process for operating a centrifugal dryer according to the present invention is proposed to solve the foregoing problems.